1. Field of the Invention
The present invention relates to semiconductor devices and methods for fabricating the same. More particularly, the invention relates to a semiconductor device which has high mechanical strength despite the use of a material having a low relative dielectric constant for forming insulation films.
2. Description of the Related Art
With recent higher integration of semiconductor devices, wiring width and wiring pitch have been reduced markedly. Since the parasitic capacitance between wires is inversely proportional to the distance between adjacent wires (wiring pitch), a reduction in wiring pitch results in an increase in the parasitic capacitance between wires. The increase in the parasitic capacitance leads to a delay in signal propagation in the wires, which is an inhibiting factor in improving the operating speed of semiconductor devices.
In order to overcome such an “increase in the parasitic capacitance between wires”, it is effective to decrease the relative dielectric constant of interlayer insulation films. That is, by using, as the material for interlayer insulation films, a material having a lower relative dielectric constant than materials commonly used, the parasitic capacitance between wires is decreased.
A porous insulation film has been noted as an insulation film having a very low relative dielectric constant. The porous insulation film is a film having many pores therein. If the “porous insulation film” is used as a material for an interlayer insulation film, the parasitic capacitance between wires can be decreased. However, under the influence of many pores present in the film, the mechanical strength of the film is decreased. As a result, it becomes difficult to provide a sufficient mechanical strength required for the device.
Under these circumstances, it has been proposed that a porous insulation film is selectively used in an interlayer insulation film. That is, in an interlayer insulation film, a porous insulation film is used only in an area that does not require strength (refer to Japanese Unexamined Patent Application Publication No. 2003-100757 [Patent Document 1] and Japanese Unexamined Patent Application Publication No. 2004-179386 [Patent Document 2]). Patent Document 1 discloses that a non-porous insulation film is provided in bonding pad forming regions so that the insulation film is not fractured during bonding, and a porous insulation film is provided in regions other than the bonding pad forming regions. Patent Document 2 discloses an example in which an interlayer insulation film having a low dielectric constant and being made of a porous material is provided only in narrow regions between wires, and a usual interlayer insulation film made of silicon dioxide (SiO2) is provided in regions other than the narrow regions.
However, in each of the techniques disclosed in Patent Documents 1 and 2, when an interlayer insulating film is formed, separate steps of forming two films made of different insulating materials (i.e., a porous insulating material and a non-porous insulating material) are required. Thus, the number of steps required for forming the interlayer insulating film is increased.